Two-photon spectroscopy (TPS), which offers selection rules complementary to those for ordinary one-photon spectroscopy (OPS), will be applied, for the first time, to study the low lying Pi electron singlet excited states of aromatic amino acids, DNA bases, dinucleotides, and DNA polymers. At the outset, polarized two-photon fluorescene excitation spectra will be obtained for aza and fluoronapthalenes for the first time. We seek from these latter experiments a critical test of a powerful theoretical scheme which makes graphic use of transition density matrices and maps to make definitive statements regarding relative one- and two-photon behavior of the two lowest Pi electron excited states of cyclic polyenes under the influence of "perturbations" such as cross-linking and inductive substitutions. With the use of CNDO/S-CI calculations and the complete transition densities, a major goal is to find a rationale for the DNA base electronic structure more explicitly linked to that used to understand simpler systems. Additionally, the fluorescence of single and double stranded viral DNA of high purity will be investigated, as well as the effect of supercoiling and polyamine induced collapse, all for the first time. Comparison of the two-photon fluorescence excitation spectrum between monomers and polymer will be carried out for DNA and DNA-protein complexes.